This invention relates to test techniques for measuring loss in optical fiber splices. It is specially suited for testing fiber splices in laser devices that are pumped via a cladding layer.
Rare-earth-doped fiber lasers are finding a variety of uses in optical communication systems where they can be integrated effectively with fiber links, and active fiber devices such as erbium fiber amplifiers. These lasers are frequently dual clad structures that are end pumped with inexpensive multi-mode lasers, such as GaAlAs. See e.g., L. Zenteno, xe2x80x9cHigh-Power Double-Clad Fiber Lasersxe2x80x9d, Journal of Lightwave Technology, Vol. 11, No. 9, pp. 1435-1446, September 1993. In a preferred structure, a threshold level of germanium dopant is incorporated into the fiber core in order to write Bragg gratings in the core and thereby create a laser cavity. Dopants such as aluminum are added to aid in solubilizing the active rare earth ions and prevent crystallization of the aluminum, and consequent scattering losses in the fiber core. See my copending patent application Ser. No. 08/908,258, filed Aug. 7, 1997, incorporated herein by reference.
The active section of a dual clad cladding pumped laser device is typically spliced to a pigtail connected to the pump laser. This splice carries relatively intense optical radiation and should be of high quality to avoid excessive losses. In prior art laser technology, the quality of the splice between the pump laser and the active laser fiber typically is not generally regarded as critical since losses at this point are not signal losses. Therefore, sophisticated splice loss measurement techniques that are available for splices in the signal path typically would not be applied to the pump pigtail splice. However, we have discovered that defective pump pigtail splices may be very significant because they often produce excessive heating in the vicinity of the splice. This heating will age the optical properties of the fiber prematurely, and may result in darkening or even melting of the fiber coating. Excessive heating is due to a variety of factors which contribute to optical absorption at the splice. Conventional optical integrating splice test apparatus will not detect the energy from absorption and excessive heating.
For optical fiber splices of acceptable quality it is important to recognize that this defect mechanism exists, and to take appropriate steps to detect unacceptable heating of the splice. We have developed a test technique for optical fiber splices that detects defective splices by measuring excessive optical absorption rather than optical scattering. Excessive optical absorption is detected by measuring heat generated at the fiber splice. The portion of the fiber that contains the pump pigtail splice is placed in a calorimeter. The pump laser is activated, and the thermal effects from optical absorption at the splice are allowed to reach equilibrium. The heat generated by this absorption is then measured. Readings above a threshold level indicate a defective splice. This technique can be used to evaluate the quality of any optical fiber splice but is especially adapted for measuring the pump pigtail splice of a dual clad cladding pump laser. This is due to the relatively high optical energy that is carried by the cladding. Defects that cause excessive optical absorption in a cladding layer are particularly harmful in this type of laser device.
The invention will be described in greater detail with the aid of the drawing.